Abstract
Crystallographic intergrowth of An 34 andesine and An 66 labradorite which can be seen with a petrographic microscope in sections cut perpendicular to [001] has been observed in amphibolite facies calcsilicate rocks of the Central Alps. The texture indicates that both plagioclases grew simultaneously in equilibrium, andesine mainly nucleating on faces (1¯10), (110), (100) and (130) of labradorite. The chemical composition measured on the U-stage and confirmed by microprobe establishes the endmembers of the andesine-labradorite miscibility gap.
As shown by X-ray and electron diffraction both endmembers have the same intermediate plagioclase structure with periodic antiphase boundaries parallel to 10¯3 and spaced 25–30 Å which is characteristic of the structure observed in andesine from igneous rocks. Two different types of domains have been observed; one is in contrast with e-reflections and the other with strong a-reflections.
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Wenk, E., Wenk, H.R., Glauser, A. et al. Intergrowth of andesine and labradorite in marbles of the Central Alps. Contr. Mineral. and Petrol. 53, 311–326 (1975). https://doi.org/10.1007/BF00382446
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DOI: https://doi.org/10.1007/BF00382446